FIELD OF THE INVENTION AND RELATED ART STATEMENT
The present invention generally relates to a technique for electrically blasting explosives, and more particularly, to a delay type electric detonator for use in a multi-step explosion.
Heretofore, in a multi-step explosion in which a plurality of explosives are fired or ignited at different times delay type electric detonators are generally used. One known delay type electric detonator comprises leg wires to be connected to a bus wire which is connected to an electric blaster, an electric igniting portion including an igniting resistor and a fuse head applied on the igniting resistor, and an electric delay circuit portion having a capacitor for storing the electric energy, an electric delay element and a switching element. At first, the electric energy is stored in the capacitor and when a given time period has elapsed after the actuation of the electric blaster, the switching element becomes conductive and the electric energy is supplied to the igniting resistor via the switching element.
In the electric detonator of the delay type, after the detonator has been assembled, that is to say after the delay circuit and igniting resistor have been installed in a housing together with the igniting resistor and explosive or the housing having the delay circuit installed therein has been coupled with an instantaneous type explosive primer, it is preferable to measure the resistance value of the igniting resistor and to confirm whether the detonator or primer can be ignited properly or not. During transportation of the detonator, the igniting resistor might be disconnected and the contact might become poor or completely broken, so that the resistance value of the igniting resistor might be increased, and further, the igniting resistance might be prolonged or shortened due to the temperature change. Under such a condition, the detonator might not explode correctly. Then, the explosion could not be carried out effectively and some detonators might not explode. This results in a serious danger. Usually the igniting resistor is made of a platinum wire and has a resistance value of about 0.6 .OMEGA.. It has been experimentally confirmed that when the resistance of the igniting resistor is increased more than 1.7 .OMEGA., the detonator might not explode. Therefore, the resistance value of the igniting resistor has to be measured with a precision of 0.1 .OMEGA..
In the known electric detonator having the delay circuit, it is impossible to measure the resistance value of the igniting resistor after the detonator has been assembled, so that it is impossible to confirm prior to actual use whether or not the detonator will explode correctly. This is due to the fact that the leg wires are separated from the igniting resistor by means of the switching element provided in the delay circuit. That is to say, if a sufficiently high voltage for making the switching element conductive is applied to the leg wires in order to measure the resistance value of the igniting resistor, the large energy is supplied to the igniting resistor via the switching element and the detonator might accidentally explod. Therefore, conduction of the igniting resistor can not be checked safely. It is considered that a separate checking terminal may be provided on the detonator or a separate checking wire may be added. However, in such a case another problem would be produced in that the detonator might be connected erroneously and the voltage might be applied to the detonator via the checking wire or terminal, and the detonator might explode erroneously.